So was the "major feature" they just hit two finger swiping, or support for font-stretch?

Actually it was the new Javascript engine (amongst other things). I don't know why you're making light of it either, it's a smart process compared to how they used to iterate. Since they started focusing on features a few at a time the browser has rapidly improved. Every change can't herald something innovative as the commercial software world well knows. This is very similar to the process Chrome used to make its rapid ascent to the top of the browser market - both in quality and market share.

The reason we have versioning numbers is to give people an idea of what's been changed. 3.0.9, for example (assuming previous version was 3.0.8) would let people know that it's a minor bug fix. 3.1.8 would let people know it's a major bug fix. 4.0.0 would let people know they've done a large overhaul on the program. By making almost every minor change to Firefox lead to a new version, they've basically thrown out that system.

It depends on your perspective. Versioning is used internally first, how external users view it is usually a secondary considersation. They haven't totally broken away from your description either, they are still issuing point releases for bug fixes for example. They have changed their development process and the versioning system was meant to reflect those changes. Instead of constant bug fix patches and long delays between major releases they iterate more quickly but base their major versions on a few important development goals at a time.

It's really improved the state of the browser too compared to where it was mired in the 4.x days. Eventually they'll likely move to background updating anyway (its already being worked on), making this all a moot point. What's really important, the number or the browser itself? They expect you to keep updating no matter, that's the mantra from all of the major browsers now. Whether that's appropriate or not is a different discussion.